Such a process is known from, int. al., M. Wilkinson and K. Geddes, “An award winning process,” Chemistry in Britain, pp. 1050-1053, December 1993. This publication describes that emulsion polymers are manufactured by addition polymerization of low molecular weight unsaturated monomers, in the presence of water and stabilizers. In the past, the polymerization was carried out as a batch process or a delayed addition batch process, but in the late sixties the closed loop continuous reactor was first developed.
A problem encountered in polymerization processes employing a tubular reactor is the formation of deposits from the reaction products on the internal wall of the reactor. These deposits lead to a need for an increased delivery pressure from the circulation pump and impair heat transfer from the reaction medium to, e.g., a coolant in a jacket surrounding the reactor tube, thus leading to higher (and often deleterious) reactor temperatures or necessitating either an increased coolant circulation rate, a lower coolant temperature, or a reduced rate of production (so as to match the heat removal).
A further general disadvantage of a fouled reactor is the reduction of volume, increasing both the recycle rate and the shear on the emulsion. This shifts the process conditions which may have been optimized on a clean reactor. In any case, product properties will drift, nullifying the advantages of consistency of production expected from continuous reactors.
One way of cleaning the inside of tubes or pipework is the use of a cleaning member or pig which is forced through the tube. German patent application 32 33 557, for instance, describes various ways of using a pig for cleaning the internal wall of a tubular reactor. In one embodiment (illustrated in FIG. 1 of the application), two ball valves are provided in the reactor tube for receiving and launching one or more pigs. The reaction product is used to drive the pig and leaves the reactor after the pig has passed the outlet. Thus, the reaction product is not and cannot be recirculated, rendering this configuration unsuitable for continuous polymerization. In a further embodiment (FIG. 3), spherical-shaped scraper pigs are passed through the pump. This method imposes severe limitations on the shape of the pig and the type of pump used. For instance, the use of a positive displacement type pump to give the low shears desirable for emulsion polymer production is not possible.
For completeness' sake, it is noted that U.S. Pat. No. 3,425,083 discloses an endless pipe which has a bent form allowing a cleaning member to circulate constantly through the pipe and past its inlet and outlet for a liquid medium. The outlet has a smaller capacity than the inlet, so that part of the liquid medium is returned from the outlet through a return portion of the pipe to and past the inlet for recycling. No means are provided for removing the cleaning member or for interrupting its circulation. Further, the pipe is intended for heating liquids like milk and not for cooling reacting emulsions. In fact, reactors are not mentioned at all.
U.S. Pat. No. 3,682,186 discloses an apparatus for by-passing scrapers or product displacers around a pipeline booster or compression station. The main line is provided with two check valves for receiving and launching the pig. Loop reactors are not mentioned.